Flexible expandable roller conveyors which utilize lazy tong support frames or structures have been widely used for many years. Lazy tongs are structures formed of a set of parallel bars, each of which is pivotally connected at its end points and midpoint to other bars in another, transversely oriented, set of parallel bars so that the structure may be extended and retracted with scissors-like action. Such structures for conveyors are disclosed in U.S. Pat. No. 3,276,558 issued Oct. 4, 1966 to Guske, et al. and U.S. Pat. No. 4,266,650 issued May 12, 1981 to Patel, et al., which are incorporated by reference. Those patents disclose conveyors in which two lazy tong structures are spanned and connected by a number of axles or rods, each of which carries several rollers. Such conveyors are sometimes known as "skate-wheel" conveyors because their rollers are similar in construction and appearance to roller skate wheels.
Lazy tong roller conveyors are particularly useful to move packages and other items from one point to other points in a plant or warehouse, and in loading or unloading trucks, airplanes, or containers. They may be flexed and extended or retracted with minimum effort to lead to the desired location, vehicle or container.
Previous lazy tong roller conveyors provide flexibility at the expense of durability, however. The axles or rods connecting the lazy tong structures and carrying the rollers are easily bent as packages or other items are thrown onto the conveyor and create great impacts and high point loads. Such items frequently impact on only one roller and thus easily deform its supporting axle. Packages or items hitting a roller from the roller's side subject the roller and axle to additional bending moments and cause further deformation.
Heavy loads sometimes cause bars in lazy tong structures to bend or buckle so that the bars themselves are an additional failure site in lazy tong roller conveyors. The bars tend to bend or buckle at their midpoints because the cross-section area at those points is reduced by the hole which connects the bar to a corresponding transversely oriented bar. Unexpected loading can cause a number of bars to buckle and the lazy tong structure to become three-dimensional and fail.
An additional source of failure in previous roller conveyors is the rollers themselves. Metal ball bearings and bearings made of other materials can suffer from a short service life when subjected to abuse of the type often encountered by these conveyors.
Previous conveyors reflect an attempt to add strength to lazy tong structures by shortening each bar in the structures. Shorter bars require that the conveyor be less flexible, however, and thus less useful when a curved path for moving packages or items is desired.